As one type of close proximity wireless communication, there is radio communication by NFC (Near field communication) using 13.56 MHz as a carrier frequency. Meanwhile, a technique for performing non-contact power supply by a magnetic field resonance method using a coil used for NFC communication has also been proposed.
In the non-contact power supply utilizing magnetic field resonance, a power transmission side resonance circuit including a power transmission side coil is disposed in a power supply device, and a power reception side resonance circuit including a power reception side coil is arranged in an electronic device as a power reception device. Resonance frequencies of these resonance circuits are set to a common reference frequency. Then, by supplying an alternating current to the power transmission side coil, an alternating magnetic field having the reference frequency is generated in the power transmission side coil. Then, the alternating magnetic field is transmitted to the power reception side resonance circuit resonating at the reference frequency, whereby an alternating current flows through the power reception side coil. In other words, power is transmitted from the power transmission side resonance circuit including the power transmission side coil to the power reception side resonance circuit including the power reception side coil.
In general, only the electronic device corresponding to the power supply device is disposed on a power supply base (a power supply mat or a power supply cradle) of the power supply device, whereby desired power supply (power transmission) is performed. However, a foreign object not corresponding to the power supply device may be erroneously placed on the power supply base. The foreign object referred to herein is, for example, an object (a card or the like) which includes a wireless IC tag having an antenna coil of 13.56 MHz that does not respond to NFC communication. Moreover, for example, the foreign object is an electronic device that has an NFC communication function which is invalid. For example, a smart phone having an NFC communication function which is turned off by software setting may become a foreign object. In addition, a smart phone having a valid NFC communication function and not having a power reception function is classified as a foreign object.
If such a foreign object is placed on the power supply base when a power transmission operation is being performed, the foreign object may be destroyed by the strong magnetic field generated by the power transmission side coil. For example, the strong magnetic field during the power transmission operation may increase the terminal voltage of the coil of the foreign object on the power supply base to 100 V to 200 V. If the foreign object is not formed so as to withstand such a high voltage, the foreign object is destroyed. Incidentally, an iron plate or the like may become a foreign object. Depending on the carrier wave frequency of power transmission, a foreign object such as an iron plate or the like may generate heat due to the magnetic field generated by the power transmission side coil.
It is essential to detect the presence or absence of such a foreign object and to take necessary measures. In this case, it is also necessary to consider the state of the electronic device as a power reception device (the reason why consideration is necessary will be explained in detail later).